y. bartosiewicz , z. aidoun, p. desevaux, and y. mercadier
DESCRIPTION
CFD-Experiments Integration in the Evaluation of Six Turbulence Models for Supersonic Ejectors Modeling. Y. Bartosiewicz , Z. Aidoun, P. Desevaux, and Y. Mercadier. Condenser. Generator. Evaporator. G. Generator. ECH. Qgen. C. D. Generator. evaporator. I. Qp. condenser. Qcond. - PowerPoint PPT PresentationTRANSCRIPT
CANMET Energy Technology Centre - Varennes
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CFD-Experiments Integration in the Evaluation of Six Turbulence Models for Supersonic Ejectors Modeling
Y. Bartosiewicz, Z. Aidoun, P. Desevaux, and Y. Mercadier
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Supersonic ejectors in refrigeration
Qp
Ejector
Generator
Condenser
Evaporator
Qgen
Qcond
Qev
A
Condenser
B
E
F
H
C
Generator
D
G
I
ECH
condenser
evaporator
Generator
Evaporator
Condenser
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Main objectives of this study
Short term:
- Assess the ability of CFD to represent the operation range of a supersonic ejector in a simple case: single phase, known properties: Air
- Choose the best suited turbulence model among those giving reasonable results in comparison to the computational cost:
- k-epsilon- Realizable k-epsilon- RNG
- k-omega and k-omega-sst- RSM
- Correctly predict some local (shocks position…) and global (entrainment rate, pressure recovery) features
Long term:
- Have a better understanding of involved phenomena (local physics, that 1-D models cannot provide)
- Set up a reliable tool for geometrical design
- Use CFD to model ejector in refrigeration with refrigerants and two phase flow
Boussinesq hypothesis
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Numerical tools
Adaptation following the pressure gradient, and y+ close to walls
- CFD package FLUENT: F.V.
- Roe flux splitting for inviscid fluxes
- Time marching technique (implicit Euler)
- Time preconditioning (for low Mach)
- Algebraic multigrid solver (block Gauss -Seidel)
-Adaptative structured-unstructured mesh
- Standard (equilibrium) wall functions
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Flow Facility (IGE*) – Computational domain*: Institute of Applied Energy, CREST-CNRS, Belfort, France
- Axisymetric computational domain
- Equivalent cross section for the secondary flow
Computational domain
m2 = 0 or constant
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Measurements (IGE): the centerline pressure
Outlet
Capillary tube
Static pressure hole
. P1
P2
Flow physics
- Probe with 1 mm external diameter
- Hole diameter = 0.3 mm
- Pressure transducer
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Comparison with experiments:Centerline pressure: without probe modeling(No secondary flow)
None of the turbulence models is able to completely reproduce shock reflections in terms of:
- Phase
- Strength
However the average pressure recovery is properly modeled.
P1 = 5 atm
m2 = 0. kg/s
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Centerline Pressure: with probe modeling(No secondary flow)
- The probe has a significant effect even though its size is small
- The numerical results are all improved with the probe modeling
- The RNG model gives the best results among k-epsilon based models and RSM
- The most important discrepancy is observed in expansions (35-50%) (condensation)
- In compressions, it is about 10%
P1 = 5 atm
m2 = 0. kg/s
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Comparison between RNG and k-omega models(No secondary flow)
- The standard k-omega model overpredicts shocks downstream the fourth shock
- RNG and k-omega-sst results comparable
- Both models give the same pressure recovery value further downstream (not shown)
P1 = 5 atm
m2 = 0. kg/s
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Other operation conditions
P1 = 4 atmP1 = 6 atm
m2 = 0. kg/s
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Measurements (IGE): the non-mixing length
Laser Tomography
* Power:1.5 kW in the blue line
* Fmirror = 300 Hz
* Light sheet with parallel edges (thickness = 0.3 mm)
* Natural marker: water droplets issued from condensation (diameter = 0.1 m)
* Additional markers: 1 m oil droplets
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Supersonic ejector operating with a secondary flow: Non-mixing length
Passive scalar equation
0 0.01 0.02 0.03 0.04 0.050
0.05
0.1
0.15
0.2
0.25
X (m)
Ideal colorant:
The laser tomography picture is treated by an image processing software to deduce lm (Desevaux et al.)
lm
0
iii
i
xxx
u
efftl
m2 = 0.028 kg/s
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Non-mixing length results
P1 (atm) 4 5 6
P2 (measured) (atm) 0.78 0.68 0.4
P2(computed) (atm) 0.61 0.52 0.4
Lm (measured) (m) 0.13 0.17 0.21
Measurements error (%) 15 12 9.5
Lm (computed) (m) k-omega 0.14 0.17 0.22
Lm (computed) (m) RNG 0.16 0.18 0.22
Error/measurement (%) (Lm)
K-omega-sst8 0 4.8
Error/measurement (%) (Lm)
RNG23 6 4.8
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Concluding remarks
* Ejector with zero secondary flow:
•RNG and k-omega-sst models provide good and comparable results.
•More discrepancies in expansions (condensation?)
* CFD-experiments integration: CFD revealed that intrusive measurement systems should be included in models for supersonic flows
* Preliminary tests conducted with induced flow have shown that the k-omega-sst model accounts best for the mixing
A wide range of operating conditions needs to be modeled with induced flow: non-shocked to shocked ejector
+ More realistic boundary conditions at the secondary inlet: total pressure
To check : - entrainment ratio: m2/m1
- local profiles
To ascertain the selection of the
k-omega-sst model